1. Field of the Invention
The present invention relates to numerical controllers, robot controllers, and programmable logic controllers which control machine tools and industrial machinery.
2. Description of the Related Art
in some manufacturing lines including a plurality of manufacturing facilities (such as machine tools, robots, and industrial machinery), controllers (such as numerical controllers, robot controllers, and programmable logic controllers) for controlling the manufacturing facilities are connected through a network, and data on the controllers is shared to perform synchronous control, cooperative control, or the like. In such a case, data are shared among the controllers through a network employing a common memory system.
In a network employing a common memory system, a predetermined region of a memory of each of controllers (nodes) constituting the network is set as a common memory region. This common memory region includes an own-node area for storing data to be transmitted from the own node to the other nodes and other-node areas for storing data transmitted from the other nodes. Further, each node generates an own-node data frame containing own-node data obtained from the own-node area in the common memory region and its own node number, and transmits the own-node data frame over the network.
A data frame of each node needs to be transmitted to all the other nodes connected to the network, and therefore broadcasting is used. The data frame transmitted by broadcasting is received by all the other nodes connected to the network. Each receiving node stores source node data obtained from the received data frame in the source node area among the other-node areas in the common memory region. This enables data sharing among all the nodes connected to the network (see FIG. 13).
In the case where each node performs broadcasting at an individual timing, a heavy load may be temporarily placed on the network. To cope with this, a technique is employed in which using a transmission right called a token, only a node holding the token transmits an own-node data frame. After the token-holding node transmits the own-node data frame, the token-holding node generates a token frame containing the number of a next token-holding node, and transmits the token frame over the network to pass the token to the next token-holding node. This is repeated to circulate the token, and all the nodes connected to the network sequentially transmit own-node data frames. When the token is passed back to the node which held the token first, data update is completed for all the nodes connected to the network. In other words, the cycle of the token determines a communication cycle.
As such a technique for circulating a token among a plurality of nodes connected to a network to share data among the nodes, JIS B 3521 discloses FL-net (OPCN-2) standardized by the Japan Electrical Manufacturer's Association.
In a network in which a token is circulated, all nodes connected to the network sequentially transmit data. Accordingly, there is the following problem: as the number of nodes connected to the network increases, the length of the circulation cycle of the token increases, and the communication cycle time of the entire network increases.
One conceivable way to reduce the number of nodes connected to the above-described network and reduce the communication cycle time is to physically divide the network. However, this method has the problem that nodes in each division network cannot share data of nodes outside the division network (see FIG. 14).
Moreover, in the above-described network, since the token circulates among all the nodes connected to the network, the data update cycles of the nodes cannot be adjusted, and the same data update cycle as that of a node which requires a short data update cycle is also applied to a node in which a long data update cycle is acceptable. Accordingly, there is a problem that needless data communications are performed to place a needless load on the network.